Introduction

      Heavy metal ions like cadmium ions are dangerous pollutants in environment [1-3].Some experimental methods for the removal of heavy elements  are used with a combination for different techniques such as membrane and electrolysis [3-5]. The extraction of heavy metal ions as pollutants from water pollution, using electrochemical methods, is important [6-9].Several metal ions in solutions can be recovered by reduction at the cathode.The recovery and extraction of metals from Ni-Cd batteries have been studied [10,11].Cadmium ion is highly toxic and responsible for poisoning the food. Binding Cd with organic compounds is a treatment for remediation of Cd in vivo and vitro [12,13]. In this work electrochemical cyclic Voltammetry behavior of nano cadmium ions in chloride form was studied.

Experimental

The used chemicals CdCl₂, KCl, Isatin are of high purity 98% of the Sigma Aldrish Company. Pure water was used after distillation. The cell has three electrodes connected to potentiostat DY2000 . Ag/AgCl ,KCl _sat was used as reference electrode, carbon glassy electrode (CGE) was used as working electrode, and platinum wire as auxiliary electrode. The electrochemical studies were done in a potentiostat of the type DY2000.Flow of purified N₂ was done to ensure diffusion experiment. The carbon glassy electrode  (CGE) is locally prepared in our laboratory from pure carbon piece and polished with fine aluminium oxide on wool piece. Area of electrode is 0.502 cm². All cyclic Voltammetry parameters  are measured at the selected two temperatures 27 and 40^oC using ultra thermostat of the type Assistant 3193.

The nano cadmium chloride (Ncc) was prepared by ball milling method ,it is technically used for reducing material particle size.This  nano cadmium chloride (Ncc) was prepared by being shaken  in a ball-mill apparatus of type Retsch MM2000 swing mill for a period of two days. The mill contains 10 cm³ stainless steel tubes and Three stainless steel balls of 12 mm diameter were used. After the ball milling process which was performed at 20225 Hz at room temperature, the particles  have a nano size. The nanoparticles were investigated using JEM-2100  TEM, Transmission electron microscope in Mansoura University.

 Results and discussion

TEM Image for nano cadmium chloride (Ncc)

Fig. 1. The TEM image of nano cadmium chloride (Ncc) from JEM-2100 TEM, transmission electron microscope.

      From this image with 72,000 X , we conclude that the nano cadmium chloride is in the form of a nano scale and dimensions of particles lie between 15.86 and 30.46 nm.

Electrochemical behavior of nano cadmium chloride (Ncc) in absence of (Isatin)

    The electrochemical behavior of nano cadmium ions in carbon glassy electrode (CGE) was examined and hemi cycle waves were obtained. Cyclic Voltammetry of cadmium ions show charge transfer at the carbon glassy electrode(CGE) in 0.1 M KCl . Ag/AgCl was used as a reference electrode to follow the redox of Cd(II) ions in aqueous solution.One cathodic peak and one anodic peak were observed according to the suggested mechanism:

Cathodic reaction   Cd ²⁺ + 2e ^-àCd

Anodic reaction      Cdà Cd ²⁺ + 2e ^-

Effect of metal ion concentrations

     Effect of cadmium ion concentrations  for nano cadmium  chloride (Ncc) was examined at two selected temperatures,26.5 and 40^oC.Cyclic voltammograms for different concentrations from 3.3 × 10^-4 till 1.96 × 10^-3mol.L^-1 in 0.1 M KCl were done . It was found that peak current gradually and linearly increases with increase in metal ion (salt) concentration due to the presence of ions active species at carbon glassy electrode(CGE) as shown in Figs. 2-4, at the two different temperatures.

Fig. 2. Different cyclic voltammograms for different nano CdCl2 concentrations in 0.1M KCl at 26.5 ˚C.

Fig. 3. Different cyclic voltammograms for different nano CdCl2 concentrations in 0.1M KCl at 40 ˚C.

Fig. 4.  The effect of temp on redox behavior of nano CdCl_2.

Fig.(4) illustrates the effect of raising the temperature on the redox behavior of nano CdCl₂. It  was observed that increasing temperature increases the peak current for the two redox waves.

 Effect of different scan rates

      Effect of scan rate of the redox behavior of and nano CdCl₂ (Ncc) in 0.1 M KCl was studied in the range 0.01,0.02,0.05 and 0.1(V.s^-1) as given in Figs.(5) &(6). The different cyclic Voltammetry analysis data were calculated and the obtained data are Ip_a (anodic current) , Ip_c (cathodic current), ∆E_P(difference in potentials), D_a ( anodic Diffusion coefficient), D_c (cathodic diffusion coefficient), E _1/2 (half wave potential), K_s ( electron transfer rate constant ), Г_a (anodic surface coverage), Г_c (cathodic surface coverage), q_a (anodic quantity of electricity) and q_c (cathodic quantity of electricity) and 𝝰na (transfer coefficient). Quasireversible mechanism was observed for the redox behavior of bulk and nano CdCl₂  (Ncc) in 0.1 M KCl  from all cyclic Voltammetry CV analysis data and specially Ip_a/Ip_c . Increase of scan rate is followed by increasing in the diffusion parameters, especially, K_s,Г_a,Г_c ,q_a and q_c indicating the increased in the diffusion process by  an increase in scan rate as the data given in Tables 1&2.

Fig. 5. Different scan rates of 1.96×10^-3 M nano CdCl₂ at 26.5 ˚C.

Fig. 6. The relation between log Ip_c and log𝜈 of 1.96×10^-3 M nano CdCl₂ at 26.5 ˚C.">

The relationship between log Ip and log 𝜈 giving straight lines indicate that the redox mechanisms are diffusion controlled for nano CdCl₂  in 0.1M KCl. Randles Sevick equation was used for the relation between peak current (anodic and cathodic) and square root of scan rate which gives straight lines. In this sense, it indicates that the redox reaction is the diffusion process.

The electrochemical behavior of bulk and nano CdCl₂ (Ncc) in presence of isatin in aqueous solution:

Effect of different isatin concentrations

Figs.(7) and (8) represent the electrochemical behavior of complex interaction between nano CdCl₂ (Ncc) and ketone (isatin) in 0.1M KCl at the two selected temperatures 26.5 and 40^oC. As shown from the previous figures by increasing the isatin concentration, the peak current decreases due to decreasing of concentration of dissolved cadmium ions at the carbon glassy electrode (CGE). Also, peak potentionl shifts to more negative values in case of oxidation and more positive value shift in case of reduction indicate complex formation.

Fig.7. Cyclic voltammograms for interaction of 1.96×10^-3 M nano CdCl₂ and different concenterations of isatin at 26.5˚C.

Fig.8.  Cyclic voltammograms for interaction of 1.96×10^-3 M nano CdCl₂ and different concenterations of isatin at 40˚C.

Figs.(7) and (8) illustrates that temperature causes more decrease in peak current (anodic and cathodic) which means that the complex formation reaction became more accelerated by increasing temperature. It is also worth mentioning that the reaction was an endothermic one.

 Effect of different scan rates

     Effect of scan rate on the interaction between bulk,nano CdCl₂ and Ketone Isatin was studied in 0.1,0.05,0.02 and 0.01 V.s^-1 Fig.(9).

Fig.9. Different scan rates of 1.96×10^-3 M nano CdCl₂ interacted with 1.63 × 10^-3 M isatin at 26.5 ˚C.

Fig.10. The relation between log Ip_c and log 𝜈 of 1.96×10^-3 M CdCl₂ interacted with 1.63×10^-3 M isatin at 26.5 ˚C.

Fig. (11) illustrates the relation between log Ip and log 𝜈 for interaction between nano CdCl₂(Ncc) in 0.1M KCl giving straight lines. Besides, it indicates the reversibility of the mechanisms and the redox mechanisms which are diffusion controlled. Randless Sevicek equation was used to apply the relation between peak current (anodic and cathodic) and square root of scan rate which gives straight lines.

Fig. 11. The relation between Ip_c ,IP_a and  𝜈^1/2  of  1.96×10^-3 M nano CdCl₂ interacted with 1.63×10^-3 M isatin at 26.5˚C.

Table 1. Effect of  different scan rates for interaction between 1.96×10-3 M nano CdCl2 (Ncc) and 1.63×10^-3M isatin(ketone) at 26.5˚C on the diffusion parameters.

Table 2. Cont. effect of  different scan rate for interation between 1.96×10^-3 M nano CdCl2 (Ncc) and 12 "> 1.63×10^-3 M isatin(ketone) at 26.5˚C on the diffusion parameters.

The equations used for the electrochemical cyclic Voltammetry calculations [15-25]

     Where ᵠ , charge transfer parameter taken as one for better approximation[13], 𝝰 charge transfer coefficient, K_s standard rate constant for electron transfer coefficient, 𝜈 scan rate, D_a diffusion coefficient for the reduced species, D_c diffusion coefficient of the oxidized species, n electrons, F faraday constant, R gas constant and T is the absolute temperature for the experiment , 𝝰 = 0.5 which can be used for a good approximation for calculations, A is the area of the electrode used [17-20].

     The complex stability constant measuring the strength and power of the interaction between CdCl₂ and isatin (Isa) is important. The complexation stability constant ( 12β"> ) nano CdCl₂ (Ncc) complexes in 0.1 M KCl are calculated by applying equation(7).

[13,16-18]

                                                       (7)

   Where (E_P)_M is peak potential for metal in absence of ligand, (E_P)_C is peak potential of the complex, R gas constant, C_L analytical concenteration of ligand (ketone) isatin (Isa). Gibbs free energies of interaction, solvation of nano  CdCl₂ (Ncc) with ketone isatin (Isa) was calculated [21,22] using equation (8).

                  (8)

Enthalpy (∆H) of complex formation reaction between nano CdCl₂ (Ncc) with isatin (Isa)  was calculated using equation (9) [17-20].

    (9)

Where  12β1">  is a complex stability constant at lower temperature T₁ (26.5 ^oC), 12β2">  is the complex stability constant at higher temperature T₂ (40^oC).

The entropy (∆S) for bulk CdCl₂ and nano CdCl₂ (Ncc) at the two used temperatures is calculated by using equation (10)

∆G = ∆H - T∆S                              (10)

Table 3. Solvation parameters for the interaction between nano CdCl2 (Ncc) and isatin (ketone) at 26.5˚C

Table 4. Solvation parameter for the interaction between nano CdCl2 (Ncc) and isatin (ketone)  at 40 ˚C

Table 5. Solvation parameter for the interaction between nano CdCl2 (Ncc) and isatin (ketone) at 40 ˚C